An I.C. (integrated circuit) device is formed by an I.C. chip capsulated within a resin package, as by a molding operation, and the chip connects to a plurality of pins (generally referred to as "leads") which project outwardly beyond the sides of the resin package. These leads are part of an elongate lead frame such that several such I.C. devices are formed longitudinally in spaced relationship therealong, with the I.C. devices thereafter being severed from the longitudinally extending edge strips of the lead frame.
As is well known, the leads are typically coated with solder, with the coating generally occurring in one of two different ways. In a first common technique, each I.C. device is separated from the lead frame, its leads are then bent downwardly substantially at right angles, and the bent leads are then dipped into a bath of liquid solder. The second conventional technique involves maintaining the plurality of I.C. devices integrally connected to the elongate lead frame so as to define a relatively flat package having a plurality of I.C. devices thereon, with this package in its entirety then being dipped into a bath of liquid solder.
The above techniques are known to possess several disadvantages. In an attempt to improve thereon, the Assignee of this invention developed various apparatus for permitting solder coating of solely the leads while maintaining a plurality of I.C. devices connected to a common elongate lead frame. In one such apparatus, rolls are disposed in pairs vertically disposed one above the other, with the lowermost rolls having their lower parts dipping into a solder bath, and the lead frames are fed horizontally through a narrow nip defined between the vertically-opposed pairs of rolls. With this apparatus, however, only one side of the lead frame can be solder coated at a time, and hence each lead frame has to be reversely oriented and again passed between the pair of rolls to coat the other side. Such is highly inefficient and undesirable.
The Assignee of this invention also developed another apparatus wherein pairs of rolls are disposed in horizontally opposed relationship and lower parts thereof dip into a solder bath. A lead frame is fed vertically through the nip between the rolls to permit simultaneous solder coating of both sides of the lead frame. In this apparatus, the rolls are rotatably driven, and the rolls act either directly on or through the viscous properties of the solder to effect driving of the lead frame through the nip. This arrangement, however, also has proven to possess operational characteristics which are less than optimum.
More specifically, in the solder coating process, an alloy component of the solder material dissipates into the metallurgical structure of the substrate metal, and at the same time metallic molecules of substrate material dissipate into the solder material which has a low melting point. For example, the solder material composed of tin or lead changes to alloys composed of tin or lead and the metal component of the substrate. Accordingly, the substrate metal surface must be heated to a temperature almost equal to the melting point of the solder material during the soldering process, although the amount of heating of the substrate metal surface is obviously different depending upon the type of solder material, the type of substrate metal and the thickness of the substrate metal.
Because of the above, and particularly when using the aforementioned apparatus wherein the lead frame is driven through the nip by the solder coating rolls, optimum soldering conditions often can not be achieved. For example, if the peripheral speed of the roll is too fast, the surface of the lead frame can not be heated to a temperature near that of the melting point of the solder material, whereby the wetability of the surface of the lead frame is insufficient. On the other hand, if the rolls are driven with too slow a peripheral speed, whereby the surface of the lead frame is properly heated so as to have good wetability, nevertheless under this situation the amount of solder material picked up by the rolls and carried upwardly into a pool at the upper end of the nip is decreased and is often not sufficient to create a sufficient coating so as to properly cover the lead frame, particularly where the leads border or join to the resin package.
The lead frame which is to be subjected to the solder coating operation conventionally comprises either a strip type or a ribbon type, each type having the plurality of resin packages mounted at longitudinally spaced intervals therealong. The ribbon-type lead frame is of great length and hence is conventionally stored by being wound on a coil, and for purposes for the present description is referred to as being "substantially continuous" since it does permit a coating operation of long duration to be carried out. The strip-type lead frame, on the other hand, is of a rather short predetermined length having a predetermined number of resin packages mounted in longitudinally spaced intervals thereon, and these strip-type lead frames are sequentially fed through the nip between the solder coating rolls. With the strip-type lead frame, however, it has been observed that excess solder tends to build up at the trailing end of the lead frame and thus creates a small bump or enlargement. This buildup of solder at the trailing end of the strip is undesirable since not only can it sometimes effect the quality of the leads disposed most closely adjacent the trailing end, but it also interferes or complicates further processing and handling of the strips.
Accordingly, the present invention relates to improvements with respect to a method and apparatus for solder coating of leads associated with a plurality I.C. devices as mounted on a common lead frame, which method and apparatus overcome many of the disadvantages associated with known methods and apparatus.
In the improved method and apparatus of this invention, a pair of solder coating rolls are disposed horizontally adjacent one another for rotation about generally parallel horizontal axes, and are slightly spaced apart to define a narrow vertically oriented nip therebetween for accommodating the lead frame therein. The lower parts of these rolls contact a solder bath, and the counter rotation of the rolls carries the solder upwardly on the periphery of the rolls through the nip, thereby creating a small pool of solder at the upper end of the nip. The lead frame is driven vertically upwardly through the nip, and the nip has a minimal dimension which is greater than the lead frame thickness. The lead frame is driven vertically upwardly by a driving device which drives the lead frame at a linear speed which is different from, and preferably slower than, the peripheral speed of the solder coating rolls. This enables a proper preheating of the surface of the lead frame due to its longer exposure to and contact with the hot solder which coats the periphery of the rolls, thereby improving the wetability of the lead frame surface. This upward movement of the lead frame through the nip and through the pool of liquid solder maintained at the upper end of the nip permits a solder coating of high quality and uniformity, and particularly of desired thickness, to be applied to the lead frame and specifically the leads.
Further, in the improved method of this invention, particularly when solder coating strip-type lead frames, the coating rolls and the strip-type lead frame are preferably independently driven at different velocities as the lead frame enters into and passes partially through the nip between the solder coating rolls. However, before the entire length of the strip has passed through the nip, such as after about one-half of the strip length as passed through the nip, then the solder coating rolls are preferably stopped while the strip continues to move upwardly at its predetermined velocity. This hence enables the pool of solder at the upper end of the nip to partially leak downwardly into and through the nip while at the same time effectively coating the remainder of the lead frame as it passes upwardly through the nip as now defined between the stationary rolls. This has been observed to substantially eliminate the bump or buildup of solder material at the trailing end of the lead frame, while at the same time permitting a proper coating of the lead frame.
Other objects and purposes of the invention will be apparent to persons familiar with methods and apparatus of this general type upon reading the following specification and inspecting accompanying drawings.